Social identity theory proposes that individuals define their self-concept based on membership in social groups, leading to ingroup favoritism and outgroup prejudice. Discuss the key concepts and empirical findings supporting social identity theory, including the distinction between personal identity and social identity, as well as the role of social categorization and social comparison processes. Using relevant psychological research and real-world examples, evaluate the implications of social identity theory for intergroup relations, conflict resolution, and collective behavior in diverse societal contexts.